Since the fifties methods have been in use to assess the compressive strength of concrete using an apparatus with a defined tip to impact upon the surface to be tested.
The best known method is the so-called Schmidt hammer that generates a defined impact energy by extending a spring and letting it drive a mallet. This mallet in turn hits upon a plunger that transfers the impact to the surface to be tested.
Upon impact the concrete is compressed and part of the energy is absorbed by plastic deformation. The remaining energy is returned and causes the plunger to rebound. The rebound is then transferred back to the mallet. The mallet then compresses the spring until the kinetic energy of the mallet is fully transferred into deformation energy of the spring. The point of this maximum compression of the spring is registered by means of a drag pointer. Its position is readable from the outside of the instrument. The reading of this instrument is expressed as R-value, meaning the maximum rebound travel of the mallet. Typically R-values range from R=20 to R=55.
FIG. 3 shows how the rebound values of the mallet can be converted to an indication of compressive strength. Note that there is a substantial influence of the angle at which the unit operates, as indicated by the three curves. When measuring inclined surfaces the angle has to be estimated and the values must be interpolated from the curve set.
In the present art the signal of interest is falsified by several error factors. Typically these can total up to 15, even 20% of the measured value. The smaller the rebound energy, the greater the percentual error contribution. Especially for rebound values less than 20 the energy absorbed by gravity and friction can be close to the rebound energy (article Dr. K. Gaede, volume 154 Schriften des Deutschen Ausschusses für Stahl-beton).
To keep the influence of friction to a minimum, the apparatus must be carefully adjusted, cleaned and inspected frequently—all factors increasing the cost of the device and leading to a limited acceptance of the rebound method.
With the advent of digital electronics and LCD displays many companies have “digitized” their instruments. Instead of having to read the position of the mechanical drag pointer, these units feature a numeric display. Up to this point such instruments have simply converted the final position of the drag pointer into an electrical value either by contacting means or non contacting (optical, Hall sensors etc.). The indicator electronics can either be a separate box or mounted right on the instrument. Such units have been in the market for over a decade.
FIG. 1 is a sectional view of a typical Schmidt hammer equipped with a linear potentiometer 1 to convert the position of the drag pointer 2 to an electrical value, which is transmitted to an external indicator unit via a connector. All the other mechanical parts are 100% identical to the original, mechanical Schmidt hammer. We note the mallet 3 that travels on the guide rod 4 and hits the plunger 5 drawn by the impact spring 6. Housing 7 and release/reload mechanism 8 are mentioned for the sake of completeness.
“Integrated” models with numeric readouts are based on standard mechanical units equipped with sensing circuitry for the drag pointer.
All these solutions suffer from the problems inherent to mechanical drag pointer indicators:
1) The rebound value is dependent on the inclination of the surface under test (effect of gravity on the mallet).
2) The readings remain dependent on the internal friction of the apparatus (mallet traveling on guide rod plus friction of drag pointer).
3) The transfer efficiency of the kinetic energy between the mallet and the plunger is not constant due to the mismatch of the two masses.
4) The impact energy (length of spring) and the zero position of each instrument have to be manually adjusted, which increases cost and the chance of maladjustments.
5) The impact energy is dependent on the angle of incidence due to gravity.
6) The readings remain dependent on the way the operator actuates the apparatus—vigorously or hesitantly (velocity of housing with respect to fixed coordinate system).
U.S. Pat. No. 5,176,026 (Leeb, Brunnner) (FIG. 2) describes an apparatus which measures the rebound travel of the mallet by means of a transducer consisting in a reflective optical detector 7a and a mallet 3 featuring grooves 8 filled with an opaque substance on its entire length. This approach eliminates the drag pointer and its friction, whereas the other error sources (effect of gravity on mallet, friction of mallet on guide rod, zero position of spring) are still affecting the result. Furthermore the reflective sensor scheme is lacking due to its susceptibility to dirt and fingerprints. This type of device has been of limited commercial success. The implementation shown in FIG. 1, although “cruder” in its design, remains the state-of-the-art.
Attempts have been made to apply a technique that is used in assessing the hardness of metals (U.S. Pat. No. 4,034,603) to the Schmidt hammer—so far these efforts have failed.
In this technique—intended for shop use—the mallet directly impinges on the sample and the housing of the impact device rests on the surface under test.
Note:
a) The Schmidt hammer is used mainly under outdoor conditions and must be sealed against dust and moisture, therefore the mallet cannot impinge directly on the surface under test, but must transfer its energy via the plunger 5. This design allows for a seal 9 between the moveable plunger 5 and the instrument housing 7.
b) The loading and trigger mechanism of the Schmidt hammer is such that the unit rests on the plunger and not on the housing of the instrument.